(a) The hue-shift problem—A common practice in the field of incremental printing, including inkjet and other liquid- or semiliquid-based technologies, is to produce secondary colors by placing one or more dots of each of plural primary colorants one on top of another, i.e. dot-on-dot printing. Ideally, colorants are conceptualized as mixing completely and forming the same color regardless of the order in which they are printed.
Unfortunately, however, when printing on practical printing media the first colorant printed immediately spreads out and penetrates into the medium. This phenomenon changes the surface and the initial conditions for the following drops.
The results include incomplete ink mixing, and asymmetrical effective concentrations of colorant. From these in turn there arises a hue shift, which depends on both the order and the timing of the dot-on-dot printing.
The order is particularly critical because a very effective way of obtaining high printing throughput is to print bidirectionally. This means, when using a scanning carriage 20 (FIG. 1) that transports printheads 23-26 across the printing medium 15, to print while the carriage is moving in each direction 16, 17 rather than just one or the other.
The desirability of high throughput emerges from the extremely competitive marketplace in incremental printers, with its intense demand for both high throughput and high image-color quality—as well as low price. Bidirectional operation answers this demand by making the most of the time needed to return the carriage for another printing pass. A nonprinting retrace or return pass at slewing speed is faster than a printing pass, but not infinitely fast.
(b) Origin of hue shift—The implication of such bidirectional operation, however, is a reversal in the order 11-14, 11′1415′ of colorant C, M, Y, K deposition—because printheads are ordinarily mounted on the carriage in a fixed order along the scan axis. Therefore, colorant deposition is correspondingly in one fixed order 11-14 when the carriage is moving in one direction—but in the opposite order 11′-15′ on retrace.
Because of the above-mentioned interactions between the colorant and the printing medium, this reversal of deposition order results in two different colors even for identical image data. This topic will require familiarity with concepts of “printmasking” and plural-pass “printmodes”, which are introduced briefly in subsection (e) below.
In a single-pass printmode, the colorant first deposited on the printing medium tends to predominate. Laying down dots of cyan 11 above dots of yellow 13, simply due to scanning rightward 17, produces a green that is biased toward the yellow.
Conversely, applying yellow 12′ above cyan 14′, due to scanning leftward 16, produces a green that is biased toward the cyan. This difference arises even though the printing medium 15 itself, and the carriage 20, pens 23-26 and colorants C, M, Y, K are all identical in the two circumstances.
In a three-pass mode the situation is more complicated. Although a slight tendency persists for predominance of the first colorant deposited, a more prominent effect is that the predominant colorant is the one with higher concentration near the medium 15.
(As will be understood, some types of colorants or media may invoke a different, and even opposite, hue-shift behavior. For example, with some inks or media, or combinations of inks and media, the predominant color may arise from the last colorant deposited—or from colorants with higher concentration far from the media. Judicious application of the principles taught this document can resolve hue shifts in those ink-media regimes as well.)
A typical conventional three-pass mode provides overlapping swaths 31-34 (FIG. 2). Due to periodic advance 18 of the print medium 15, each swath (e.g. 32) is stepped relative to its two nearest neighbors (e.g. 31, 33) by one-third of their common heights. Each swath 32 therefore overlaps those two neighbors 31, 33 by one-third of that common swath height.
To understand how this generates a complex hue shift, consider the two portions 35, 36 of an image designated region 1 and region 2. Region 1 (upward hatched in the drawing) is formed from swaths made in three passes 31-33, namely the bottom or leading one-third of the rightward-scanning 17 first-pass swath 31, the center one-third of the leftward-scanning 16 second-pass swath 32, and the top or trailing one-third of the again-rightward-scanning 17 third-pass swath 33.
In summary, region 1 has three sets of colorant layers, with alternating directionalities 17, 16, 17 respectively. Region 2 (downward hatched) is instead formed from swaths made in three passes 32-34 of oppositely alternating directionalities 16, 17, 16.
These are the bottom or leading one-third of the leftward-formed 17 second swath 32, the center one-third of the rightward-formed 16 third swath 33, and the top or trailing third of the leftward-formed 17 fourth swath 34. Both sequences are bidirectionally alternating, and both deposit exactly the same amounts of the same colorants; but the two sequences start with different scan directions 17, 16 respectively.
To simplify the description slightly, this discussion will first focus attention on only the construction of a green 40g (FIG. 3) colorant lamination formed by cyan C and yellow Y colorants. The bottom two layers, a C layer above a Y layer, are formed by the bottom, leading third 43 of the rightward 17 first-pass swath; therefore after the first pass the pattern 61 is simply “CY”, reading from the top layer downward toward the printing medium.
The next two layers, a Y above a C, are formed by the central third 42 of the leftward 16 second-pass swath. Therefore after the second pass the aggregated pattern 62 is “YCCY”, again reading from the topmost colorant layer downward toward the print medium.
The final two layers, again a C above a Y, are formed by the top, trailing third 41 of the rightward 17 third-pass swath, so that the final aggregated pattern 63, still reading through the laminations from the top down, is “CYYCCY”. Green 40g (the portion above region divider 47) is accordingly formed as just that pattern 41g-42g-43g. The fourth pass 34 does not print in the first region 35 at all.
Now for comparison the behavior in region 2 is built up in the same way from three passes—but now they are the second, third and fourth passes respectively, so as mentioned earlier the alternations are opposite in order. Starting with the bottom one-third 46 of the leftward second pass, this sequence continues with the central one-third 45 of the rightward third pass, and concludes with the top one-third 44 of the leftward fourth pass.
Immediately after the first pass 31 (FIG. 2) there is no colorant in region 2, as the second pass 32 is the earliest one to provide any colorant in that region 2. After the second pass 32, region 2 has a top-down colorant-deposition pattern 54 (FIG. 3) of just two installments “YC”.
After the third pass the aggregate colorant-deposition pattern 55 is “CYYC”. After the fourth, the pattern 56 is “YCCYYC”, forming green 40g (the portion below region divider 47) as a pattern 44g-45g-46g. 
Now it is possible to directly compare the two colorant installment patterns:
region 1region 2CYYCYCCYCYY C.In region 1, although there are two Y installments in a row (the second and third installments), physically those two deposits of yellow may be regarded as merging into simply a thicker layer of yellow.
The same is true for the two Y installments in region 2 (the fourth and fifth installments). The analogous observation holds for the two C installments in each region.
Taking these merging phenomena into account, the overall sequence can be simplified by looking at colorant layers rather than installments:
region 1region 2CYYCCYY C.This tabulation makes clear that colorant sequences in the two regions even if considered disregarding the number of installments in each colorant layer, are fundamentally different.
These tabulations show that colorant nearest the printing medium (the bottom of the tabulation) is opposite for the two regions. To the extent that the concentration of colorant nearest the medium tends to control apparent hue more strongly than colorant elsewhere (as is often theorized), these two colorant-layer patterns show that there is an intrinsic problem to be solved.
On the other hand, the colorant farthest from the medium (the top of the tabulation) is also opposite for the two regions. To the extent that the concentration farthest from the medium tends to control apparent hue more strongly than colorant elsewhere (as is also sometimes theorized), the tabulated patterns affirm that there is still an intrinsic problem to be solved. Hue is accordingly biased toward cyan in one of the regions and toward yellow in another.
Based on this presentation, it can be appreciated—without repeating the foregoing full development—that red 40r colorant layers similarly are formed as “MYYMMY” in region 1 (the aggregate of patterns 41r-42r-43r) and as “YMMYYM” in region 2 (from patterns 44r-45r-46r). The perceived hue is therefore biased toward magenta in one region and yellow in the other. Similarly a viewer sees blue 40b as magenta-biased in one region, but cyan-biased in the other.
This example is based upon a three-pass printmode. Any successful method for removing or minimizing the hue shift must be usable and effective in printmodes with very few printing passes or so-called successive “installments” of colorant deposition. This condition too flows from marketplace pressure for high throughput.
(c) Direct mitigation of hue shift—Heretofore, favored approaches for tackling the hue-shift problem have been relatively direct—either printing unidirectionally, or using printmodes that couple a certain number of printing passes with half that number of print-medium advances. The latter repeats each swath in both print directions, i.e. proceeding with a sort of limp, using N passes with N/2 advances.
Both these approaches actually physically eliminate the hue shift by removing its underlying causes. In principle these solutions are extensible to printmodes with few passes, but they are disadvantageous in that they significantly degrade throughput. As mentioned above, full bidirectional printing is the fastest way to complete an image, and the limping mode represents a throughput compromise.
In addition, the second of these approaches is susceptible to an artifact known as boundary banding. This is true because the limping mode doubles and thereby aggravates the undesirable deposition, within very short times, of relatively large amounts of ink along the edge of a swath. Although ordinary amounts of boundary banding may be mitigated by the methods introduced in the Gil and De Peña patent documents mentioned above, some small degree of image distortion may arise when boundary banding is doubled as suggested here.
A third direct approach uses symmetrical pen configurations, as suggested for example in Japanese patent publication 58215351 (1983) and U.S. Pat. No. 4,593,295 (1986)—both of Matsufuji Yoji et al.—and also U.S. Pat. No. 4,528,576 of Nobutoshi Mitzusawa et al. Such symmetrical constructions force drop orders in both print directions to be identical, by duplicating the occurrence of each colorant in a symmetrical way around a central reference pen—e.g., magenta-cyan-yellow-cyan-magenta (MCYCM).
This approach may present a perfect solution in the sense of maintaining full throughput without hue shift, but requires major mechanical changes. In general it is likely that for such a printer, the electrical connection system must be augmented and the processor configured.
Further, the carriage must be slightly wider, leading to enlargement of guiderails and the drive belt, codestrip and overall case. This last-mentioned change in turn impacts the cost of packaging, shipping, and inventory (storage).
For those users who are particularly concerned about the very highest color quality and fidelity, such relatively small increases are likely to be acceptable—and it is certainly not intended to criticize this solution. Users who are less demanding, however, may object to the results of these added cost elements.
It may be difficult to determine what fraction of all users may be so inclined. Hence the overall cost effectiveness of symmetrical pen configurations is uncertain.
(d) Indirect mitigation of hue shift—An indirect approach does not tackle the hue-shift problem at its source but instead attempts to conceal it. One such approach is use of bidirectional printmodes with a high number of passes, particularly eight and more.
This tactic provides drop-order statistics sufficient to keep the hue-shift effect below the threshold of human perceptibility. As noted above, however, modernly use of printmodes with high numbers of passes is disfavored—and reduction of the number passes exposes the effect. Image quality achievable with few passes is poor, especially for odd numbers of passes.
(e) Modern printmasking—As mentioned above, heretofore printmasking has been associated with hue-shift mitigation only to the extent of camouflaging the hue effect by dilution in a relatively large number of passes. Printmasking, as implied by the discussion above and as well known in the incremental-printing field, enables laying down in each pass of the pens only a fraction of the total ink required in each section of the image—so that any areas left white in each pass are filled in by one or more later passes.
This tends to control bleed, blocking and cockle by is reducing the amount of liquid that is all on the page at any given time, and also may facilitate shortening of drying time. In fact multipass printmasking tends to hide not only hue shift but also a great variety of other undesirable artifacts in incremental printing; unfortunately, however, multipass printmodes are very slow.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a “printmode”. Some printmodes such as square or rectangular checkerboard-like patterns tend to create objectionable moire effects when frequencies or harmonics generated within the patterns are close to the frequencies or harmonics of interacting subsystems. Such interfering frequencies may arise in dithering subsystems sometimes used to help control the paper advance or the pen speed.
One particularly simple way to divide up a desired amount of ink into more than one pen pass is the checkerboard pattern mentioned above: every other pixel location is printed on one pass, and then the blanks are filled in on the next pass.
To avoid horizontal “banding” problems (and sometimes minimize the moiré patterns) discussed above, a print mode may be constructed so that the paper advances between each initial-swath scan of the pen and the corresponding fill-swath scan or scans. As illustrated in section (b) above, this can be done in such a way that each pen scan functions in part as an initial-swath scan (for one portion of the printing medium) and in part as a fill-swath scan.
Because regular printmasking patterns themselves create new artifacts, it was for several years believed that random or randomized masking would be the key to eliminating repetitive artifacts. This belief suffered from two misunderstandings.
First, even masks formed by entirely random processes generate extremely objectionable repetitive patterns, and indeed sometimes quite bizarre ones, when tiled (stepped repeatedly across and down) for use in a sizable image. As taught in the Garcia patent documents mentioned earlier, this defect arises when individual masks are small—meaning, ordinarily, smaller than roughly two to three centimeters (one inch) across.
Second, even large masks when made randomly produce unacceptable images. This phenomenon follows the well-known capability of truly random sequences to include long repetitive runs.
Thus, merely by way of example, there is a nonzero probability that a random-number generator will generate twelve zeroes in a row. The same is true for nine sevens, eleven thirteens, etc. Of course such coincidences arise in meaningful quantities only in very large streams of numbers—but such very large streams are the norm when considering image data, which typically run to several million colorant-pixels in a standard magazine-size page.
In the aggregate all these probabilities come up to a rather common occurrence of clumping in numerical arrays. In images printed with masks generated by random-number techniques, such clumping manifests itself as granularity, graininess, particularly in image highlight regions.
The remarkable Garcia documents show both how to avoid tiling of too-small unit masks and how to control the degree of randomness in mask generation, to obtain an ideal tradeoff between repetitive-mask artifacts and graininess. Garcia accomplishes this by running programs, entitled “Shakes”, that can generate a usable mask at each attempt—even on the fly, just ahead of print-engine operations, in real time.
He conditions these program operations by directing the programs to obtain critical constraints as parameters from an easily accessible configuration file. The constraints include so-called “neighborhood” constraints for controlling e.g. the relative proximity of pixel positions printed in the same pass or immediately successive passes.
The term neighborhood is understood as three dimensional, to encompass pixel-addressing opportunities that are “near” one another in terms of numbers of passes (i.e. time) as well as more simply in terms of the pixel grid on the print medium. The Garcia programs—like incremental printers in general—can operate in software or in firmware, or even in hardware (application-specific integrated circuits or “ASICs”). The configuration file is advantageously kept simple and open so that a system designer can modify it straightforwardly.
To enable adjustment of the relative amount of randomness, the configuration file accepts constraint values for interpretation as probabilistic weighting factors. In the Shakes regime these values are central to actual performance, as Shakes defines almost all operations in terms of relative probabilities or preferences rather than absolute constraints—thereby allowing the program enough degrees of freedom to find an actual mask solution in every attempt. (It will later be seen that this property of Shakes also protects that printmasking system against certain aspects of the present invention that could otherwise cause Shakes itself to fail.)
The Garcia developments include, in addition to the basic Shakes programs, a two-stage strategy that meets all the nozzle weighting requirements within an acceptable processing time. His mask generation incorporates a first so-called “precooking” process that is plot independent, performed just once; and then a later so-called “cooking” or “popup” process that is performed before each plot and if desired can be conditioned by metrics developed from the character of the plot itself.
The precooking process creates a preference-sorted set of mask texture candidates, depending only on nozzle neighborhood conditions—defined for each set of maskbuilding constraints. The cooking or popup process selects one of the various available precooked mask levels.
Cooking also replicates those levels in such a way that the different printheads do not print onto the same pixel in the same pass. Eventually the cooking phase takes into consideration nozzle-weighting features, firing frequency and mask parity restrictions corresponding to each printhead.
Heretofore the Garcia systems have not been associated with solutions to the hue-shift problem.
(f) Conclusion—Thus persistent problems of hue shift in bidirectional operation have continued to impede achievement of uniformly excellent inkjet printing—at high throughput. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.